Effect of hydrogen addition on low and high frequency turbulent combustion dynamics predicted using fuel-forced Scale Adaptive Simulations

The effect of hydrogen enrichment on the combustion dynamics of a partially premixed swirl-stabilized methane flame is studied. The simulated test rig was built to investigate low-frequency combustion dynamics found in industrial furnaces. The low frequencies are facilitated by a long upstream tube. Using fuel-forced Scale Adaptive Simulations, the Flame Transfer Function (FTF) is calculated, describing the flame’s response to fuel mass flow rate fluctuations generated at the location of the fuel injector. Three hydrogen-methane blends are investigated: 100 vol% CH 4 , 40/60 vol% CH 4 /H 2 and 10/90 vol% CH 4 /H 2 at 50 kW thermal power and 0.62 global equivalence ratio. To reduce the computational cost of the FTF retrieval, the fuel inlet is forced by a superposition of four sine waves. With increasing hydrogen content, the flame height is observed to reduce from 60.0 mm to 45.7 mm for 60% H 2 and 31.9 mm for 90% H 2 . This directly influences the low-pass filter-shaped FTF gain and phase: the cut-off frequency increases from 290 to 500 Hz and the convective time delay decreases from 2.8 to 1.5 ms. It is shown that the FTFs of different fuel blends collapse to the same curve when scaled using the Strouhal number based on the convective time delay. This allows for the calculation of the FTF for any blend based on a steady-state solution and a reference FTF. Next, the FTFs are applied in an acoustic network model. The model shows that for the low frequencies found due to the long upstream tube (below 70 Hz), the difference in time delay constant for different H 2 content does not have a significant effect on the predicted eigenfrequencies. For higher frequencies, this difference becomes larger relative to the mode period. Due to this and the higher cut-off frequency, the modes become more unstable and shift to higher frequencies with increasing hydrogen content.